This application claims priority under 35 U.S.C. xc2xa7xc2xa7 119 and/or 365 to Appln. No. 199 59 243.8 filed in Germany on Dec. 8, 1999; the entire content of which is hereby incorporated by reference.
The invention relates to a fuse for the medium-voltage and high-voltage areas. Such fuses are used to interrupt overcurrents, such as those which occur as a result of short circuits.
U.S. Pat. No. 4,638,283 discloses a fuse of this generic type which has a core with a cruciform cross section in a housing, which core is composed of electrically insulating material and around which fuse elements in the form of strips are wound in the form of a helical line. The core is fitted axially with rings which follow one another spaced apart and which each have a depression where they are crossed by a fuse element, in which depression a combustible body is arranged which contains an igniter, for example composed of KClO4, in particular with a boron additive, and, apart from this, is composed of a combustible material, for example aluminum, KClO4 and silicagel as a binding agent. An ignition circuit which connects the connections to one another and is composed of a wire with a relatively high resistance connects each row of combustible bodies which follow one another axially, where the wire passes through the igniter. An element which evolves gas and is composed of hydratized aluminum silicate, boron phosphate and a binding agent, and optionally zirconium, is also fitted on the fuse element. The housing is filled with an extinguishing means, for example quartz sand. If a fuse element melts at any point, considerably more current flows through the ignition circuit, so that the combustible body is caused to ignite, and the fuse elements are interrupted at a row of points which are spaced axially apart from one another.
A fuse of similar construction is disclosed in U.S. Pat. No. 4,486,734. There, the combustible material is a mixture of a metal such as Zr, Hf, Th, Al, Mg as a material which burns, an oxidant such as KClO4 or some other perchlorate or chlorate, and a binding agent such as silicagel.
In a further fuse of similar construction (U.S. Pat. No. 4,319,212), incisions on the outsides of the ribs of the core, over which fuse elements run, are filled with a combustible material, for example with a mixture of aluminum or magnesium oxide and a filler such as mica, glass fiber, asbestos or quartz powder, and a binding agent such as polyester.
In another fuse (EP-A-0 641 005), the core is coated with a combustible material which is composed of a material which evolves gas, such as guanidine carbonate or acetate, diphenylguanidine, a guanine, melamine, hydantoin or allantoin derivative or a cyanurate and a film-forming polymer such as polyurethane, acrylic, melamine or melamine formaldehyde resin as a binding agent. Furthermore, it may contain hydratized aluminum, CaCO3, boric acid or Mg(OH)2 as an additive.
According to EP-A-0 657 910, the housing of an otherwise similar fuse is filled with a sand whose grains are coated with a mixture of a combustible material, which is composed of a gas-evolving material such as guanidine, guanidine carbonate or acetate, 1,3-diphenylguandine, guanine, melamine, urea, hydantoin or allantoin, and a binding agent such as urethane, melamine, epoxy resin, polyester or, in particular, acryl.
According to U.S. Pat. No. 4,099,153, a fuse wire is wound in the form of a helical line around a core composed of four parallel rods and is clamped against one of the rods in each case by means of a number of brackets, which are composed of a material that emits gas and are pushed axially onto the core, following one another spaced apart.
U.S. Pat. No. 3,705,373 discloses a fuse having a fuse element in strip form, which is weakened by holes at a number of successive points, and at one of these is also weakened by a layer of material with a low melting point, such as Sn or Cd. A wire with, for example, an aluminum core and a Pd sheath is located parallel to this. If the fuse element melts through at any point, then so much current flow through the wire that the aluminum core melts and reacts explosively with the Pd sheath, as a result of which the fuse element is interrupted at a number of points.
A common feature of all the described fuses is that they respond only when at least one fuse element has melted through. They are thus suitable only for interruption of large overcurrentsxe2x80x94in general greater than approximately three times the rated currentxe2x80x94which cause such melting through. However, smaller overcurrents cannot be interrupted by such fuses, or at least not reliably and sufficiently quickly.
The invention is based on the object of specifying a fuse which responds quickly and reliably, and interrupts the current reliably, even in response to small overcurrents.
The fuse according to the invention responds well before a fuse element melts through. Once it has responded, its resistance rises dramatically, thus resulting in rapid interruption of even relatively small overcurrents, in particular in the critical range between 1.1 and three times the rated current.